Due to improved combustion, the amount of PM (particulate matter) in exhaust gas emitted from modern internal combustion engines has greatly decreased. However, PM emission standards are becoming more stringent requiring a further decrease in the number of emitted PM particles larger than 50 nanometers.
In this case, a DPF (diesel particulate filter), etc., is installed in the exhaust gas passage of an internal combustion engine. However, depending upon the operating conditions of the internal combustion engine, installation of a DPF often does not satisfy PM emission standards.
However, regarding the formation of PM in an internal combustion engine, it has been known that fine particles contained in the fluid, e.g., intake air, fuel, engine oil, etc., fed into the combustion chamber of the internal combustion engine, serve as nuclei accelerating the formation of PM. That is, even if particles in the fluid fed into the combustion chamber are smaller than 50 nanometers, they serve as nuclei in the combustion chamber and become PM particles larger than 50 nanometers, which cannot satisfy the requirement of emission standards of PM particles.
In order to further decrease emission of PM, it is necessary to decrease particles smaller than 50 nanometers contained in the fluid that is fed into the combustion chamber of an internal combustion engine.
However, in conventional devices for purifying exhaust gas of internal combustion engines, means for decreasing the emission of PM is based on improving the combustion in the internal combustion engine and treating the exhaust gas. There is no prior example of decreasing the number of PM by preventing particles smaller than 50 nanometers that serve as nuclei of PM from entering into the combustion chamber.
For example, the filters disclosed in JP-A-2004-301121 and Japanese Patent No. 3249051 are traditional air filters for filtering relatively large solid matter (sizes on the order of microns) from the intake air for protecting the internal combustion engine, while the filter disclosed in JP-A-10-238330 is for removing contaminating substances for the exhaust gas emitted from the crankcase. Although all of these filters are described as high efficiency filters, they do not remove particles smaller than 50 nanometers that serve as nuclei of PM.